1. Field of the Invention
This invention relates to magnetic disks having a plurality of tracks and means for locating individual tracks.
2. Description of Prior Art
A typical magnetic disk drive includes a magnetic disk comprising a disk-shaped, nonmagnetic substrate and a film of magnetic media formed on each surface of the substrate. An example of such a disk is disk 10 illustrated in FIGS. 1a and 1b. Data is recorded on and read off of the media on the top surface of disk 10 by a read/write head 12. Disk 10 is connected to a rotor shaft 13 of a spindle motor 14 (FIG. 1b) which rotates disk 10 relative to read/write head 12. Motor 14 is typically either a 2-phase motor or a 3-phase motor. A typical 3-phase motor is illustrated in FIG. 2, and includes 4 magnets 70a to 70d mounted on a rotor housing 72 (which is rigidly affixed to rotor shaft 13), 9 coils 74a to 74i, and three hall sensors 76a to 76c. Rotor housing 72 and magnets 70a to 70d rotate relative to coils 74a to 74i and hall sensors 76a to 76c . The north pole of magnets 70a and 70c face coils 74a to 74i, while the south pole of magnets 70b and 70d face coils 74a to 74i. Each rotation cycle, the north pole of magnets 70a and 70c and the south pole of magnets 70b and 70d pass over each of hall sensors 76a to 76c. The hall sensors are used to determine when motor coils 74a to 74i should be driven with current. Disk drives employing a 2-phase motor operate in a similar manner.
As disk 10 rotates, read/write head 12 hovers over the portion of magnetic disk 10 passing underneath read/write head 12. Read/write head 12 senses changes in the magnetic field caused by the data recorded in the magnetic disk to thereby read the data. In addition, read/write 12 head can generate a magnetic field to record new data in magnetic disk 10. (As can be seen in FIG. 1b, a second magnetic disk 16 is also typically mounted on rotor shaft 13. Additional read/write heads 18, 20 and 22 are provided to read data from and write data to media on the bottom surface of disk 10, the top surface of disk 16, and the bottom surface of disk 16, respectively.)
Data is typically recorded in a series of tracks or concentric circular regions, e.g., tracks 0 to N on the media surface. A typical 31/2 inch diameter disk can have as many as 600 to 800 tracks, each track comprising either 17, 18 or 32 sectors, depending on the software executed by the host CPU coupled to the disk drive.
Read/write head 12 is mounted on an arm 15 which is rotatably mounted on a support 24. Arm 15 is also mechanically coupled to a capstan 28 of stepper motor 26. As capstan 28 rotates, arm 15 (and therefore head 12) is moved either towards or away from the center of magnetic disk 10, i e., either in the direction of arrow C or arrow B. In this way, read/write head 12 can be positioned to read data from or write data to any of the tracks on magnetic disk 10.
When the magnetic disk drive is first turned on, it is necessary to ascertain the position of read/write head 12 relative to the various tracks of the magnetic disk. This is typically done in one of a number of ways. For example, one technique is to have stepper motor 26 pull arm 15 in direction B until elbow 30 of arm 15 hits a mechanical stop (not shown). The track underneath read/write head 12 when arm 15 impacts the stop is defined as track 0 and all other tracks are positioned relative to track 0. Unfortunately, the parts of the disk drive which mechanically impact each other tend to wear out and become unreliable. Further contamination particles are generated due to wear. Also, the impacting parts cause undesired noise.
Another technique for determining the location of track 0 is to employ an optical sensor (not shown). Thus, to find track 0, arm 15 is moved in the direction of arrow B until arm 15 blocks light from reaching the optical sensor. Unfortunately, this technique is unreliable. For example, if dust or dirt is present on the optical sensor, it will not be able to detect the position of arm 15.
Another technique for locating track 0 is to provide a track -2 which contains a unique, high frequency data pattern. During reset mode, read/write head 12 is moved in direction B until read/write head 12 senses the unique data pattern. Once that unique data pattern is found, the head moves in direction C two steps to thereby be positioned over track 0.
It is also necessary to determine the location of the beginning of each track, i.e., the location of the first sector of data on each track. In one prior art device, an additional magnet 32 is mounted on rotor shaft 13 or rotor housing 72 of motor 14 (FIG. 1b). An additional hall sensor 34 is affixed to the disk drive housing near the path traced by magnet 32 as motor 14 causes magnet 32 to rotate. At the point in time when magnet 32 passes next to hall sensor 34, hall sensor 34 generates an electrical signal which determines where the first sector of each track is.
It would be desirable to eliminate magnet 32 and hall sensor 34 to thereby minimize the number of components required to manufacture the disk drive. Although, as mentioned above, spindle motor 14 includes three other hall sensors which provide pulses indicating when the spindle motor is in a certain position, the three other hall sensors provide positive output pulses twice per revolution and negative pulses twice per revolution (because of the magnetic fields provided by magnets 70a to 70d), and could not be used to differentiate whether the disk was in a first position or a second position 180.degree. out of phase with the first position.